AD2S80A: Noise on DIR output

Q

The DIR output has a lot of noise on it. What could be the cause of his noise?

A

The DIR output is asynchronous, so in theory it can change in between LSBs even
though the direction of rotation does not appear to have changed.
However this would usually only be seen if there are very small changes in the
direction of rotation i.e. less then 1lsb. This is unlikely to be the cause of
the significant noise which you are referring to.
The DIR output is derived from the velocity signal. This can be observed on the
Integrator output pin. This output voltage is converted to current through the
R6 resistor and then fed back into the VCO input. This is then passed through
comparators to determine the actual direction. If there are noise/spikes on the
VCO input this could trip the comparators but may not be enough to affect the
current input to the VCO which defines the position output. You should start by
checking for noise on the VCO input.